1. Field of the Invention
The invention relates to a method for manufacturing a semiconductor light emitting device. In particular, the invention relates to a method for manufacturing a semiconductor light emitting device that emits light from a light emitting element through a wavelength conversion layer, mixes the excitation light that is emitted from the light emitting element with wavelength-converted light from the wavelength conversion layer, and then emits the mixed light to the outside.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. 2004-111882 discloses an example of a conventional LED used as a wavelength conversion type semiconductor light emitting device, which is shown in FIG. 1.
Namely, according to the disclosure in Japanese Patent Laid-Open Publication No. 2004-111882, an LED 1 includes two lead frames 2 and 3, an LED chip 4, a first resin layer 5, a second resin layer 6, and a resin mold 7. The LED chip 4 is positioned on the bottom face of a cavity 2a formed on the upper end of the lead frame 2. The first resin layer 5 is filled inside the cavity 2a up to the upper surface of the LED chip 4. The second resin layer 6 is filled over the first resin layer 5. The resin mold 7 is formed so as to enclose both the upper ends of the lead frames 2 and 3.
The LED chip 4 can be a blue LED chip, for example. The LED chip 4 is placed onto and then die bonded to the bottom face of the cavity 2a (lead frame 2). The chip 4 is wire bonded to the other lead frame 3 using a wire 4a, such as a gold wire. Thereby, the chip 4 is electrically connected to both the lead frames 2 and 3.
The first resin layer 5 and the second resin layer 6 are prepared by mixing an optically transparent resin with particles of a fluorescent material, which act as a wavelength conversion material. Further, the concentration of the fluorescent material in the first resin layer 5 is set to become lower than the concentration of the fluorescent material in the second resin layer 6.
Further, the concentration of the fluorescent material of the first resin layer 5 and the second resin layer 6 is distributed such that the product between the optical path length for light from the LED chip 4 that reaches the upper surface of the second resin layer 6 and the concentration of the fluorescent material is almost constant.
According to this type of LED configuration, when light emitted from the LED chip 4 in each direction passes through the first and second resin layers 5 and 6, the light will pass through an optical path where an almost identical amount of fluorescent material exists. Consequently, uniform light emitting characteristics without any uneven color can be obtained on the upper surface of the second resin layer 6 by mixing the color of the light that has undergone wavelength conversion by the fluorescent material with the color of the original light that does not pass through the fluorescent material.
The following problems occurred in LEDs with this type of configuration. Namely, the optically transparent resin is mixed with the fluorescent material and hardened when forming the first and second resin layers 5, 6. It is a known fact that for this case, the heavier fluorescent particles will precipitate by gravity due to differences in the specific gravity of both layers. Because of this, the concentration of the fluorescent material within the resin cannot be made uniform in both the first and second resin layers 5 and 6.
Surface tension also occurs in the optically transparent resin between the cavity 2a and the LED chip 4. Because of this surface tension, it is difficult to make the surface shape of the first resin layer 5 uniform. Consequently, it is difficult to make the product between the optical path length and the concentration of the fluorescent material almost constant. This in turn makes it difficult to achieve uniform light emitting characteristics without any uneven color.
In addition to the LED chip 4 itself generating heat, the fluorescent material emits heat due to energy loss during wavelength conversion. The concentration of the fluorescent material after precipitation is not high, thereby resulting in inefficient and poor dissipation of heat from the fluorescent material through the LED chip 4 to the lead frame 2.
These problems not only exist in blue LED chips but similarly exist in LED chips which emit other light and semiconductor light emitting devices, such as LEDs, which emit a mixed color of light consisting of or including light from other light emitting elements and wavelength-converted light from a wavelength conversion material.
Further, these problems not only occur with LEDs of the type in which the lead frame, provided with a cavity, is insertion molded. For example, the same type of problems occur in semiconductor light emitting devices of the type in which a flat lead frame is insertion molded in a resin housing, a cavity is formed on the upper surface of the resin housing, and one portion of the lead frame is exposed to the bottom of the cavity. As another example, the problems also exist in semiconductor light emitting devices, such as LEDs, in which the cavity is formed on the upper surface of a semiconductor substrate. The LEDs are also provided with electrode layers composed of a conductive thin film that wraps around from the bottom face of this cavity up to the upper surface of the substrate through the sides of the cavity and, according to circumstances, down along the side surfaces of the substrates to the rear surface thereof.